1. Field of the Invention
The present invention relates to a scan driver and, more particularly, to a scan driver and a scan driving system with low input voltage, and their level shift voltage circuit.
2. Description of Related Art
Currently, in flat panel displays, thin-film transistors (TFTs) on a display panel are controlled by a driving unit essentially consisting of a data driver (source driver) and a scan driver (gate driver). The scan driver sequentially turns on TFTs of each column on the display panel through an internal output signal, so as to concurrently charge the column's display points (pixels) to corresponding required voltages and thus present different gray levels.
However, a voltage threshold VT for a transistor in a typical silicon process is approximate to 1 V and lower, and that for a TFT in a low temperature polycrystalline silicon (LTPS) TFT process is from about 2.5V to about 5V. For a 2V range of noises under such a high threshold, an input signal has to be greater than 4.5 V (2.5V+2V) to turn on TFTs, in order to gain a higher input voltage to drive the TFTs, resulting in consuming more power.
In related patents, U.S. Pat. No. 5,646,642 granted to Maekawa, et al. for a “Circuit for converting level of low-amplitude input” has disclosed a level shift circuit which can have a lower input voltage and a higher output voltage. However, it still needs more power because two current sources are applied. This cannot meet with strict requirement for power consumption. Therefore, it is desirable to provide an improved scan driver with low input voltage such as 3.3 V to drive TFTs on the display panel, so as to mitigate and/or obviate the aforementioned problems.